Biocrust-specific contribution to soil organic carbon under contrasting grazing intensities in grasslands of Mexico
Globally, semiarid grasslands are important stocks of soil organic carbon (SOC). In these ecosystems, diverse biocrust communities colonize vegetation-free interspaces between grasses. Photosynthesizing biocrusts consist of associations of cyanobacteria, algae, lichens, and mosses. With their anchoring structures (rhizines, polysaccharides, etc.) biocrusts bind soil particles and thus potentially contribute to SOC in this biome. In Mexico, severe trampling by domestic livestock has modified the cover, composition and functioning of biocrusts. However, it is unknown whether various species/groups of biocrusts are differentially affected under contrasting grazing intensities. We compared the contribution to SOC among cyanobacteria and diverse lichen species at different soil depths in open C4 grasslands in Jalisco, Mexico. We collected samples (5 x 5 cm) of biocrusts and associated soil at three depths (0-0.5 cm, 0.5-1.5 cm and 1.5-3.0 cm). We quantified the contribution to SOC by cyanobacteria and lichens (both with the C3 photosynthetic pathway) to different soil layers using information on natural abundance of 13C stable isotopes in biocrusts, C4 grasses and soils and applying isotope mixing models. In case of intensive and seasonal grazing systems, we compared two and four abundant lichens, respectively. Under moderate grazing and in grazing exclosures, we compared one lichen species with cyanobacteria.
Under intensive grazing, Acarospora socialis contributed significantly more SOC (43-69%) than Diploschistes diacapsis (30-50%) (P<0.0001) at all depths. Under seasonal grazing, the relative contribution to SOC at all soil depths depended on lichen species (P<0.007). While SOC overall was greater in A. socialis than in A. obpallens, it did not change with increasing soil depths for both species. SOC in association with D. diacapsis dropped from 38 to 18% (P=0.01) and for A. schleicheri from 53 to 25% (P=0.0004) between 0.5 and 3.0 cm depths. No species-specific differences in relative SOC contribution at a given soil depth occurred, except for A. socialis. Under moderate grazing, SOC contributions by cyanobacteria and Psora icterica were similar at each depth (P<0.01), however SOC declined by 20% (cyanobacteria; P<0.01; and P. icterica; P<0.0001) from 0.5 to 3.0 cm depth. In grazing exclosures, cyanobacteria and D. diacapsis did not differ in SOC at any soil depth (P>0.3). In both cases, SOC dropped by close to 20% (P<0.0001). Particular grazing intensities differentially affected the relative contribution of SOC, yet this response was group/species specific. We estimated that biocrust communities may contribute about 3 tCha-1 in the top 3 cm of soil in grazed semiarid grasslands.